The present invention relates, in general, to a protective device, and more particularly to a protective device for, for example, an electric motor of a type having a main body, which carries windings, and at least one sensor, which is arranged as passive motor element upon a winding and outputs electric signals to an evaluation unit.
An example of a sensor includes a temperature sensor which is embedded in the windings and is monitored by an evaluation unit or transducer card to ascertain or prevent inadmissibly high temperatures. Operating voltages for the sensor or the transducer card are typically in the range of safety low voltages of about 5 V to 24 V. On the other hand, there are motors for machine tools which operate at voltages of about 600 V which may rise to 2000 V as a consequence of capacitive effects or oscillations. Thus, it becomes important already during normal operation to isolate the sensor from the windings. However, localized fire or a meltdown of the insulation may be encountered when the windings are short-circuited, so that the winding voltage may breakdown via the sensor to the evaluation unit or transducer card. Not only may this lead to a destruction of the involved components but there is also a risk to an operator in view of vagabond voltage that may reach the operating elements.
One proposal to prevent a voltage breakdown involves the use of stronger insulation for the sensor. This is disadvantageous because the sensor reacts very sluggishly to temperature fluctuations so that a rapid temperature increase cannot be detected with sufficient speed. Another approach involves the use of thermistor-type triggering devices for motor protection which cut, just like a relay, a connection between a sensor and an evaluation unit. As these devices are relatively bulky and typically operate at a voltage of 230 V, they are normally mounted in a switch cabinet or to the inverter of the motor.
It would be desirable and advantageous to provide an improved protective device which obviates prior art shortcomings and which is simple ands compact in structure and yet reliably protects evaluation units from overvoltages generated by the sensor.